Cascaded polyolefin slurry polymerization employing disengagement vessel between reactors

ABSTRACT

A disengagement vessel employing a lock hopper effectively reduces concentration of non-polymer-associated components in the polymer product slurry of a first olefin slurry polymerization reactor, allowing cascading of a second slurry polymerization reactor operating at lesser concentration of comonomers, hydrogen, and other components to produce multicompositional polyolefin polymers and/or polymers having a multimodal monomer distribution, e.g. diblock polymers having substantially non-overlapping comonomer contents between the blocks.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the use of cascaded slurry reactors topolymerize olefins to produce polyolefin homo- and copolymers ofmultimodal molecular weight distribution and/or composition.

2. Background Art

Slurry reactors are in widespread use for production of polyethylenehomo- and copolymers. Slurry reactors include stirred tank reactors andwater-jacketed tubular reactors arranged in a series of continuoushorizontal or vertical loops. A “slurry solvent” in which polyethylenehas low solubility constitutes the continuous phase in such reactors,and in the case of slurry loop reactors, is driven around the loop atrelatively high speed by one or more rather massive pumps. Ethylene,supported catalyst, comonomers, and processing additives are injectedinto the loop where polymerization takes place, creating a slurry ofpolyethylene in solvent. A plurality of settling legs allow polymerparticles to partially sediment out, creating a slurry of higher solidscontent, which is released periodically to harvest polymer. Slurryprocesses are widely used throughout the world. It has recently beenproposed to cascade slurry reactors to produce multimodal resins.

Multimodal (including bidmodal) polyolefin resins are desirable due tothe improved processability associated with a lower molecular weightfraction, and superior physical properties associated with a highermolecular weight component. See, e.g., U.S. Pat. No. 6,346,575 in thisrespect. Polyolefin resins which contain blocks of differentcomposition, whether due to differences in properties such as long chainbranching, or due to differing monomer content, are also desirable inmany applications. Such polymers may be described as having amulticompositional (including bimodal, or “diblock”) monomerdistribution.

Multimodal resins may be prepared by physical blending two or moreresins having different molecular weight distributions. One disadvantageof such blended resins is that blending constitutes an additionalprocess step. Moreover, the blending must be performed in such a waythat a homogenous product is obtained. The blending operation not onlyadds additional cost to the resin, but moreover, resins produced byblending have generally inferior physicochemical properties as comparedto multimodal resins having been produced by “in situ” routes.

Preparation of polymer blends in situ avoids physical blending and itsdisadvantages. Four types of in situ multimodal polymer production maybe conceptualized. In a first process, a single reactor is employed withtwo distinctly different catalysts, each catalyst prepared separately onits respective support. One catalyst is selected to provide a highermolecular weight product than the other catalyst. In such a process, twodistinctly different polymers are created, and the product is distinctlyheterogenous. Such products are generally inferior in their processingproperties, especially for applications such as film production.

In a second process, a single reactor is again used, but two differentcatalysts are contained on the same support, i.e., so-called “dual site”catalysts. As a result, two different polymers grow from the samecatalyst particle. The resultant polymer may be described as“interstitially mixed.” A much greater degree of homogeneity in thepolymer product is thus obtained at the expense of more complex catalystpreparation. Although this process offers advantages in capital andinstalled costs relative to multi-reactor processes, the design andsynthesis of dual site catalysts is difficult. An additional processdisadvantage is that use of a single reactor reduces the number ofprocess parameters that can be manipulated to control polymerproperties.

In a third process, cascaded reactors are employed, and additionalcatalyst is added to the second reactor. The polymer particles from thefirst reactor continue growth in the second reactor, although at aslower rate. However, new polymer growth begins on the newly addedcatalyst. Hence, as with the first process described, a heterogenouspolymer product is obtained, with the same deficiencies as describedpreviously for such products.

In a fourth process, cascaded reactors are again employed, but catalystis added only to the first reactor. The supported catalyst associatedwith the first reactor polymer contain further active sites whichinitiate polymerization in the second reactor. The second reactorpolymerization parameters are adjusted to establish a differentpolymerization rate and/or molecular weight range as compared to thefirst reactor. As a result, an interstitially mixed polymer is obtained.

EP-A-0057420 represents an example of a cascaded slurry process whereincatalyst is introduced only into the first reactor. However, molecularweight is regulated by the presence of hydrogen in both reactors, withthe second reactor having higher hydrogen concentration than the firstreactor, thus limiting the types of interstitially mixed polymers whichmay be produced. Polymerization at lower hydrogen pressure in the secondreactor is not possible. In addition, the polymer formed in each reactoris limited to a specific weight percentage range relative to the weightof the final product.

U.S. Pat. No. 5,639,834 (WO 95/11930) and published application WO95/10548 disclose use of cascaded slurry reactors in which the catalystfeed is also limited to the first reactor. In both references, the firstreactor polymerization is conducted at very low hydrogen concentration,and all olefin comonomer is incorporated within the first reactor. Thesecond polymerization is conducted at high hydrogen concentration withno comonomer feed. U.S. Pat. No. 5,639,834 additionally requires thatthe takeoff from the first reactor be by way of a settling leg.Continuous takeoff is said to produce inferior products. These processesdo not allow operation of the second reactor at lower hydrogenconcentration than the first reactor. Moreover, limiting olefincomonomer incorporation to only the first reactor limits the types ofpolymers which may be produced.

WO 98/58001 alleges that significant advantages in polymer propertiesare achievable by conducting a two-stage polymerization, the first stageat high hydrogen concentration and low comonomer concentration, and thesecond stage at low hydrogen concentration and high comonomerincorporation. The reactor may be a single reactor or a cascaded reactorsystem, the latter being preferred. A single catalyst, introduced intothe first reactor, may be used. Lower hydrogen concentration in thesecond stage is achieved by limiting the choice of catalysts to thosewhich rapidly consume hydrogen. Cessation of hydrogen feed thus causesthe hydrogen concentration to fall rapidly between stages. The inabilityto add significant comonomer to the second stage or to increasecomonomer incorporation in the first stage detracts from the ability toproduce a wide variety of polymers. Moreover, the catalyst choice islimited to those which consume hydrogen, when a single catalyst is used.

U.S. Pat. Nos. 6,221,982 B1 and 6,291,601 B1 disclose cascaded slurrypolymerizations where at least two distinct catalysts are employed. InU.S. Pat. No. 6,221,982, a Ziegler-Natta catalyst is employed in thefirst reactor with high hydrogen concentration and no or low comonomerincorporation. A hydrogen-consuming catalyst with low olefinpolymerization efficiency is introduced downstream into the firstreactor product stream. As a result, hydrogen is consumed prior toreaching the second reactor, wherein the polymerization is conducted atsubstantially zero hydrogen concentration. The second stage employssignificant olefin comonomer. U.S. Pat. No. 6,291,601 is similar, butemploys a metallocene catalyst in the first reactor.

Both the U.S. Pat. Nos. 6,221,982 and 6,291,601 processes, as well thatof WO 98/58001, are inefficient in both monomer usage and thermalloading, since the hydrogenation reaction consumes ethylene, producingethane by hydrogenation. In addition to the increased thermal loadingcreated by this reaction, the ethane produced is an inert gas which mustbe purged from the system. Moreover, in the U.S. Pat. Nos. 6,221,982 and6,291,601 processes, an additional relatively expensive hydrogenationcatalyst which contributes little to polymer production must be added.Finally, all three processes require substantially homopolymerization inat least the first reactor, thus limiting the types of polymers whichmay be produced.

U.S. Pat. No. 6,225,421 B1 discloses use of cascaded reactors whereinethylene is homopolymerized in the presence of hydrogen in a firstreactor, hydrogen is physically separated from the first reactor productstream, and the product is copolymerized with 1-hexene and additionalethylene at reduced hydrogen concentration in the second reactor.However, the patent contains no disclosure of any apparatus suitable forremoving hydrogen from the first reactor product stream. Moreover, thenecessity to restrict the first polymerization to homopolymerization islimiting.

It is also desirable to produce polymers having a block configuration.One block may be different from another due to greater or lesser longchain branching, for example, or the blocks may be different due todifferent comonomer incorporation. In cascaded reactors, it is difficultto obtain a sharp delineation between blocks due to transfer ofmonomers, catalyst, etc. from the first reactor into the second. Forexample, if a diblock copolymer having a first block derived fromcopolymerizing ethylene and 1-butene and a second block derived fromcopolymerizing ethylene and 1-hexene, butene in the exit stream of thefirst reactor will cause the second block to contain butene as well ashexene.

It would be desirable to use series-configured slurry reactors whereinhydrogen is introduced into a first slurry reactor to produce a lowmolecular weight first polymer, following which this first polymer thenintroduced into a second reactor operated at lower hydrogenconcentration, without the requirement of employing a catalyst whichspecifically encourages hydrogenation. It would further be desirable toprovide a cost-effective apparatus suitable for removing hydrogen fromthe product stream of a first reactor operating at higher hydrogenconcentration than a second reactor in series with the first. It wouldbe yet further desirable to provide a hydrogen removal process which canaccommodate comonomer incorporation in any reactor of the reactorbattery. It would also be desirable to provide a process where anysoluble or gaseous component can be effectively removed or reduced tolow concentration prior to the entry of the first reactor product streaminto a subsequent reactor, especially comonomers, such that multiblockpolymers may be produced where the comonomer content of a second orsubsequent block may be selected independently of the comonomer employedin the first or prior reactor.

SUMMARY OF THE INVENTION

It has now been surprisingly discovered that hydrogen and othercomponents which are not associated with polyolefin particles may beeffectively removed from the product stream of a first polyolefin slurrypolymerization reactor through means of a disengagement vessel equippedwith a lock hopper for harvesting a polyolefin polymer slurry havingsignificantly reduced concentration of “non-polymer-associatedcomponents.” The process preferably removes hydrogen from the firstslurry reactor. The lock hopper contents are flushed intermittently intoa subsequent polymerization reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a disengagement vessel inaccordance with the subject invention.

FIG. 2 illustrates schematically one embodiment of cascaded slurryreactors employing an intermediate disengagement vessel to lowerhydrogen concentration in the product slurry from a first slurry reactorprior to its introduction into a second slurry reactor.

FIG. 3 illustrates one embodiment of a lock hopper useable with adisengagement vessel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thus, the present invention pertains to a polyolefin polymerizationprocess wherein at least two slurry reactors are employed to producemultimodal and/or multicompositional polyolefin resins, preferably aprocess wherein a first reactor employs hydrogen to limit polymermolecular weight, and a second reactor employs a reduced level ofhydrogen or no hydrogen to produce a higher molecular weight polymer. Byhydrogen is meant diatomic hydrogen. In the case where three or moreseries connected reactors are employed, the hydrogen concentration in a“prior” reactor will be higher than that in a “subsequent” reactor. Theprocess may also be used to eliminate or substantially reduce soluble orgaseous components other than hydrogen from a prior reactor, i.e. othernon-polymer-associated components. Thus, the two reactors, even thoughcascaded in series, can operate substantially independently.

The process is highly flexible, and allows free choice of comonomerincorporation in each reactor. Moreover, while not necessarilydesirable, the process allows for use of multiple catalysts among thevarious reactors. Catalyst choice is virtually unlimited. In addition,other reactors may be operated in parallel, for example but not bylimitation as parallel feeds to a single subsequent reactor.

Slurry processes for olefin polymerization are well known, and aredescribed, for example, in PROCESS ECONOMICS PROGRAM REPORTS 185 and185A (2000). Additional details may be found in the patents citedpreviously, herein incorporated by reference, and in many other patents,publications and treatises. These and other references also disclosenumerous catalysts, modifiers, etc., which can be used in slurrypolymerization processes.

In a preferred embodiment of the present invention, the hydrogen contentof a first reactor product stream is lowered to a lower concentration bymeans of a disengagement vessel (“DV”) for hydrogen (“HDV”), generallyof elongate construction, equipped with a lock hopper for producttransfer at a lower end thereof. The remaining details of the slurrypolymerization process are conventional and well known to those skilledin the art.

The DV of the present invention may be best described with reference toone embodiment thereof as illustrated in FIG. 1. The DV body 1 in thisembodiment is an elongate tank 2 terminating at a bottom portion into aconical end 4. Entering the body 1 from the top (side entry is of coursealso possible) is prior reactor product stream 6, optionally heated byheater 7. At the bottom of the DV is lock hopper 10, having polymerinlet 11, solvent flush inlet 12, and polymer slurry outlet 13. In thisembodiment, a shot feeder 15 is located between the DV conical end 4 andthe interior cavity of lock hopper 10. An optional nitrogen sparge inlet16 is located near the bottom of the DV. The DV is equipped with a levelcontrol line 17 associated with a level detector 18 which maintains aconstant fluid level. The DV is also equipped with a pressure sensor 19and a vapor discharge line 20. An optional fresh solvent line 5 addsfresh solvent to the incoming prior reactor product stream. Valves 12 aand 13 a control the flushing of the lock hopper. Operation of pressureand level controls is well known to those skilled in the art.

In operation, the DV is preferably operated at the bubble point of thesolvent, at an elevated temperature with respect to the prior reactor.For example, when the solvent is isobutane and the prior reactoroperates at 700 psig (ca. 49 bar) and 180° F. (82° C.), the DV mayadvantageously operate at 500 psig (ca. 35 bar) and 210° F. (99° C.).The prior reactor product stream may be heated, for example by steam, tothe higher temperature desired, the DV may be surrounded with a heatjacket, or any combination of these or other means of heating may beemployed to establish the desired operating temperature.

The prior reactor product stream preferably enters the DV through aninlet 21, positioned below the surface of liquid in the DV to minimizeturbulence. Polymer particles begin to settle toward the bottom of theconical end 4 of the DV. The lock hopper has been filled with solvent,preferably at a somewhat lower temperature than the DV slurry. Thepressure of solvent in the lock hopper should be substantially the sameas that in the vessel above. If the pressure cannot be adjusted easilyto be substantially the same by other means, a pressure equalizationline 22 may communicate with the vessel to ensure equal pressure. Thisline may be valved by control valve 23 during emptying of the lockhopper. The shot feeder may also incorporate a dump line 23 to empty thelock hopper of solvent during shut down, maintenance, etc. The shotfeeder is rotated to an open position, connecting the DV interior withthe lock hopper interior. The heavier polymer particles flow down intothe lock hopper, displacing the preferably cooler and hence densersolvent in the lock hopper, which then flows upward countercurrent tothe settling polymer particles, washing them of first reactor solvent.The more dense solvent in the lock hopper has little tendency to mixintensely with the less dense, higher temperature solvent, thisdisinclination toward mixing at the bottom of the conical end of the DVbeing assisted by introduction of a nitrogen sparge which is preferablyintroduced at or above the area where the lock hopper solvent and DVslurry solvent meet.

Thus, when hydrogen disengagement is desired, for example, the lockhopper, initially full of solvent having zero hydrogen concentration,gradually receives polymer particles which have been washed bydisplacement of upward flowing solvent. When the lock hopper containsthe desired solids level, the shot valve is closed, and the lock hopperinlet 12 a and outlet 13 a valves are opened. Solvent flushes thepolymer slurry from the lock box, and fills the lock box with fresh,hydrogen-free solvent. The cycle is then repeated. The polymer slurryfrom the lock hopper, now substantially depleted of hydrogen, isintroduced directly or indirectly into a subsequent slurry reactor. Thissame process may be used effectively to disengage other prior reactorcomponents, such as olefin comonomers.

The second or subsequent reactor also, in general, has a finite hydrogenconcentration. In many cases, therefore, the polymer slurry contents ofthe lock hopper may be directly input into the subsequent reactor. Ifnecessary, the pressure of the polymer slurry may be increased byconventional methods prior to its entry into the subsequent reactor. Ingeneral, the hydrogen in the polymer slurry feed to the subsequentreactor will constitute less than 10% of the total hydrogen feed to thesubsequent reactor, and is preferably less than 5%, more preferably lessthan 2% of the fresh hydrogen feed, on a mol/mol basis.

Should no hydrogen or only a very low hydrogen feed be fed to thesubsequent reactor, the low level of hydrogen in the polymer slurry ofthe lock hopper may still be too high for effective control of polymerparameters, particularly melt index, in the subsequent reactor. In sucha case, the lock hopper slurry outlet stream may enter a second HDVoperating in the same manner, to further lower hydrogen concentrationprior to entry into the subsequent reactor. Alternatively, the polymerslurry from the lock hopper may enter a flash drum or series of flashdrums where solvent is flashed away at reduced pressure, hydrogen beingalso removed in the solvent flashoff. The polymer slurry discharge fromthe flash drum may then comprise an inlet stream to the subsequentreactor. By “direct” introduction into a subsequent reactor is meantthat the polymer slurry is not further treated to remove hydrogenfollowing its exit from the HDV.

Vapor and/or liquid exiting the DV in response to level and/or pressurecontrol is treated in the conventional manner as with other streams ofthe overall polymerization process to remove and recycle solvent,unreacted monomer, etc. For those streams where separation isimpractical, the various streams may be burned for their fuel value.

FIG. 2 illustrates a simplified diagram of a cascaded polyolefin slurrypolymerization process employing a disengagement vessel of the subjectinvention. FIG. 2 illustrates two continuous loop slurry reactors 30 and31. The first slurry loop reaction is shown with three feed lines 34, 35and 36, although a greater or lesser number may also be used. These feedlines feed solvent, monomer, comonomer (when used), hydrogen, catalyst,catalyst activators and water scavengers, antistats, etc., either withdedicated inlets for each, or combination inlets. From one loop ofslurry sloop reactor legs, a product takeoff leg 38, which may be of thecontinuous type, or a single or one of a plurality of conventionalsettling legs, provides a polymer slurry from reactor 30 to the hydrogendisengagement vessel 40 as previously described. Fresh solvent isoptionally added to the product takeoff leg at 41, while gases exit theDV at 43. Hydrogen disengagement is augmented by a nitrogen inlet(sparge) 44, and settling polymer particles collect in the lower portion45 of DV 40, and flow intermittently into solvent-filled lock hopper 50.The polymer is flushed intermittently from the lock hopper by openingand then closing flush valves 52 and 53, valve 52 flushing the polymerfrom the lock hopper through valve 53 by means of fresh solvent 51. Theoutlet slurry flowing through line 54 serves as an inlet stream tosubsequent reactor 31, along with additional inlet streams 55, 57 and58, which, for example, may feed additional olefin monomer, comonomer,and solvent, and optionally but not preferredly, additional catalyst,activator, scavengers, and the like. Product takeoff from reactor 31 ispreferably by means of one or more conventional settling legs 60. Theproduct slurry is treated conventionally to remove solvent and unreactedmonomers, i.e., by a series of product separation flash drums.

FIG. 3 illustrates in detail a stainless steel lock hopper which hasbeen used with a 30 inch (76 cm) diameter disengagement vessel, locatedbelow shot feeder 15 and disengagement vessel 1. The lock hopper 10 isconnected to the shot feeder by a 3 inch by 2 inch (7.5 cm×50 cm)reducer 76. The lock hopper 10 consists of a 2 inch (5.0 cm) pipe tee70, connected to a 2 inch by ¾ inch (5.0 cm×1.9 cm) eccentric reducer 71by standard connecting flanges. At the right of tee 70 is a flangedclosure 72 through which a ½ inch (1.27 cm) solvent flush pipe 73passes, terminating at about the near T-juncture of the tee 70. Entry offlush solvent into the solvent flush pipe 73 is controlled by ½ inch(1.27 cm) ball valve 74. Discharge of polymer slurry from the lockhopper 10 is through discharge ball valve (¾ inch; 1.9 cm) 75. Thechoice of components was made to take advantage of standard components.In a commercial embodiment, corresponding components would likely befabricated. Also shown schematically is optional pressure equalizationline 77 and dump line 78.

The solvent used in the process of the invention may be any solvent orsolvent mixture conventionally used, for example hexane or heptane, butis preferably a light solvent such as propane, butane, or isobutane.Isobutane is preferred. By the term “light solvent” is meant aparaffinic solvent having a boiling point less than 0° C. under standardpressure.

The predominant olefins to be polymerized (“monomers”) include ethylene,propylene, 1-butene, 2-butene, etc. Ethylene and propylene are mostpreferred. Suitable comonomers are unsaturated compounds, preferablyolefins, other than the predominant olefin monomer. Thus, when ethyleneis the monomer, propylene may be a comonomer, and vice versa. Preferredcomonomers for polyethylene copolymers, in addition to propylene,include 1-butene, 1-hexene, 1-octene, 1-decene, 1-dodecene, norbornene,cyclohexene, and dienes such as 1,3-butadiene. For polypropylenecopolymers, preferred comonomers include ethylene and those of C₄ orgreater carbon content listed as comonomers for polyethylene polymers.The comonomers listed above are exemplary, and not limiting, and may beused in mixtures as well. Other unsaturated monomers such as styrene,(meth)acrylates, (meth)acrylic acid, vinyl halides, vinyl ethers, vinylesters, maleic and fumaric acids and the like may be used in minoramounts along with the olefin monomers and comonomers. Preferably, onlyolefin monomers and comonomers are used.

The reaction is generally catalyzed by transition metal complexcatalysts, including the so-called “Ziegler-Natta” catalysts, chromecatalysts, and single site catalysts, for example optionally substitutedcyclopentadienyl and other n-bonded titanium, zirconium, and hafniumcomplexes such as bis(methylcyclopentadienyl)zirconium dichloride andbis(cyclopentadienyl)hafnium dimethyl. Cocatalysts or activators such asnon-coordinating bulky anions, metal alkyls, or alumoxanes are oftenuseful and generally required. The catalysts are generally suppliedsupported on inorganic carriers, preferably silaceous carriers such assilica, although homogenous catalysts may also be useful. All thesecatalysts, as well as other additives such as antistats, antifoulantsand the like, are well known to those skilled in the art of olefinpolymerization.

Because hydrogen disengagement occurs between slurry reactors in thereactor cascade, it is not necessary to select catalysts withsubstantial hydrogenation activity. Rather, the catalyst can be selectedwith regard to desired polymer properties such as molecular weight,molecular weight distribution, degree of short and long term branching,randomness and/or efficiency of comonomer incorporation, etc. Therelative freedom with respect to catalyst choice allows for productoptimization not possible when only hydrogenating catalysts arerequired. By the same token, such catalysts may be used, either alone orin conjunction with other catalysts when desired polymer propertiesdictate their use.

The DV and its associated components may be fabricated of conventionalmaterials, for example carbon steel, Hastelloy® alloys, Inconel®,nickel, stainless steel, and the like. However, stainless steel ispreferred. Clad reactors may also be used. The volume of the DV may varysignificantly depending upon the output (Kg/hr) of the first slurryreactor; whether a single DV is used or whether two or more are operatedin parallel, each fed by the product slurry from the first reactor; andwhether a second DV is operated in series with a first or whether an DVoutlet stream is subject to further removal methods. For a pilot plantfirst slurry reactor having a 44 gallon (166 L) capacity and a nominal 2hour residence time for which hydrogen disengagement is desired, a DV of30 inch (46 cm) inside diameter and length (to conical bottom) of about50-60 inches (1.3 m to 1.5 m) (170 gallon, 643 L) is satisfactory whenemployed as the sole hydrogen removal means in cascaded reactors.

With the foregoing in mind, sizing the various stream flows and makeup,and operating temperatures and pressures can be established byestablished methods, for example with assistance of industry standardsoftware. Proprietary software based on basic chemical engineeringprinciples may of course be used as well.

The lock hopper volume is generally about 5% or less of the DV volume,although larger lock hoppers may be used if desired. The DV may serve asthe supply to a single lock hopper or to a plurality of lock hoppers.The lock hoppers themselves may constitute a simple tube of any desiredcross-section with appropriate shot feeders or valves between the DV andthe solvent supply and product flush lines, or may constitute a singlevalve having multiple passageways which alternate between communicatingwith the DV and with the solvent lines.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLE 1

A pilot plant cascaded reactor configuration is employed to producepolyethylene copolymers. The first reactor is a slurry loop reactorhaving a volume of 44 gallons (166 L), while the second reactor is an 88gallon (232 L) slurry loop reactor. The settling leg of the firstreactor is directed to a 170 gallon (643 L) disengagement vessel asshown in FIG. 1. The lock hopper of the disengagement vessel is that ofFIG. 3. The overall process is therefore similar to that shown in FIG.2.

Ethylene is copolymerized with 1-butene in the presence of hydrogen toproduce a high melt index polymer in the first reactor (“A reactor”)employing a titanium Ziegler-Natta catalyst activated withtriethylaluminum, and with higher 1-butene concentration andsubstantially no hydrogen in a second reactor (“B reactor”), to producea multimodal polymer of low melt index. No additional catalyst is addedto the second reactor. The reactant concentrations and productproperties are tabulated in Table 1. Although a very small amount ofhydrogen is fed to the second reactor to reproducibly control meltindex, the actual hydrogen concentration in the second reactor is belowthe detectable limit, despite the 1.0 mol % hydrogen present in thefirst reactor. The disengagement vessel is quite effective indisengaging hydrogen between the two reactors.

EXAMPLE 2

Example 1 is repeated with somewhat different monomer amounts in thefirst reactor, and with both 1-butene and 1-hexene comonomers in thesecond reactor. Once again, hydrogen is very effectively disengaged bythe disengagement vessel between the reactors. The details are presentedin Table 1.

EXAMPLE 3

Example 1 is repeated with slightly different monomer and hydrogencontent in the first reactor, but with the second reactor employing1-hexene as the comonomer. The details are presented in Table 1. As canbe seen, although 1.7 mol percent butene is employed in the firstreactor, the disengagement vessel removes the majority of butene as wellas hydrogen, allowing the polymer produced in the second reactor toincorporate only a very minor amount of butene. The hydrogendisengagement vessel is effective to remove both hydrogen and comonomerfrom the first reactor. TABLE 1 Example 1 2 3 A Reactor ReactorTemperature, ° F. 180 (82.2) 180 (82.2) 180 (82.2) (° C.) Ethylene Conc.mol % 4.7 4.2 4.3 Butene Conc, mol % 1.7 1.7 1.7 Hydrogen Conc, mol %1.0 1.0 0.9 Melt Index, g/10 min 62 71 70 Density, g/cm³ 0.954 0.9550.954 B Reactor Reactor Temperature, ° F. 180 (82.2) 190 (87.8) 180(82.2) (° C.) Ethylene Conc, mol % 8.4 8.8 9.3 Butene Conc, mol % 2014.6 0.1 Hexene Conc, mol % n/a 5.95 6.1 Hydrogen Feed Rate, pph 0.000750.00065 0.002 Hydrogen Concentration, Below DL¹ Below DL¹ Below DL¹ mol% Melt Index, g/10 min 0.06 0.10 0.07 Density, g/cm³¹⁹ 0.931 0.927 0.944¹DL—detectable limit

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1-18. (canceled)
 19. A polyethylene copolymer of multimodal molecularweight distribution, comprising an interstitially mixed blend of apolyethylene polymer of a first molecular weight and a polyethylenepolymer of a second molecular weight higher than said first molecularweight, comprising: a) a first polyethylene polymer prepared bycontinuously polymerizing ethylene and one or more copolymerizablecomonomers in light solvent at a first hydrogen concentration in a firstslurry reactor; and b) a second polyethylene polymer prepared bycontinually polymerizing ethylene and optionally one or morecopolymerizable comonomers in the presence of said first polyethylenepolymer, in the absence of additional olefin polymerization catalystsand at a hydrogen concentration lower than said first hydrogenconcentration, wherein said olefin polymerization catalyst is not arapidly hydrogen-consuming catalyst.
 20. The polyethylene copolymer ofclaim 19, wherein at least one comonomer selected from the groupconsisting of propene, 1-butene, 1-hexene, 1-octene, 1-decene,1-dodecene, 1-norbornene, and 1,3-butadiene is employed preparing saidfirst polyethylene polymer, and the same or different comonomer isemployed preparing said second polyethylene polymer.
 21. A polyethylenepolymer prepared by a) first polymerizing ethylene and at least onecomonomer in a first slurry reactor optionally containing hydrogen toform a first polymer slurry; b) substantially removing said at least onefirst comonomer from said first polymer slurry and optionallysubstantially removing hydrogen, if present in said first slurryreactor, to form a comonomer-depleted slurry; c) introducing saidcomonomer depleted slurry into a second slurry reactor, introducing acomonomer different from said first comonomer; and d) recovering aninterstitially mixed polymer having blocks of different comonomercontent.
 22. The copolymer of claim 19, prepared by the process ofpolymerizing ethylene and at least one copolymerizable monomer in afirst slurry reactor; introducing a non-polymer-associatedcomponent-containing polymer slurry from a first slurry reactor into adisengagement vessel, said disengagement vessel terminating at a lowerend thereof with a lock hopper having a cavity therein which isselectively communicatable between an interior of said disengagementvessel and a supply of slurry solvent; filling said cavity with slurrysolvent when said cavity is not in fluid communication with saiddisengagement vessel; causing said cavity to be in fluid communicationwith said interior of said disengagement vessel, and allowing polymerparticles contained in a polymer particle slurry in said disengagementvessel to enter said cavity, displacing slurry solvent into saiddisengagement vessel from said cavity; ceasing communication of saidcavity with said interior of said disengagement vessel, establishingfluid communication of said cavity with said supply of slurry solvent,and flushing polymer particles from said cavity with slurry solvent as asolvent flush polymer slurry, at least one non-polymer-associatedcomponent concentration in said solvent flush polymer slurry being lessthan the concentration of said at least one non-polymer-associatedcomponent in said disengagement vessel; introducing said solvent flushpolymer slurry into a second slurry polymerization reactor andpolymerizing ethylene, optionally with one or more copolymerizableolefins, in said second slurry polymerization reactor; and recovering amultimodal polymer from said second slurry polymerization reactor. 23.The copolymer of claim 22, wherein the interior of the disengagementvessel is maintained at a pressure and at a temperature such that saidslurry solvent is at its bubble point.
 24. The copolymer of claim 22,wherein a liquid phase in said disengagement vessel is maintained at apressure lower than the pressure of said first slurry reactor and at atemperature higher than the temperature of said first slurry reactor.25. The polymer of claim 21, wherein a single olefin polymerizationcatalyst is employed, and no additional catalyst is added to said secondslurry reactor.
 26. The polyethylene polymer of claim 21, wherein atleast one comonomer selected from the group consisting of propene,1-butene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-norbornene, and1,3-butadiene is employed preparing said first polyethylene polymer, andthe same or a different comonomer is employed preparing said secondpolyethylene polymer.
 27. The process of claim 21, wherein said firstcomonomer is butene.